Valve gear of the type known as direct-acting valve gear employs tappets having one end thereof contacting the engine cam shaft with the other end of the tappet in direct contact with the end of the stem of the combustion chamber valve. Direct acting valve gear offers the advantage of low mass, fewer working parts and higher stiffness due to the elimination of the rocker arm and/or push rods. Low mass and high stiffness result in a high natural resonent frequency which allows the valve gear to attain higher rpm's before valve mis-motion occurs. Direct acting valve gear also permits the use of lighter valve spring loads for a given valve motion and engine speed as compared with those used in other valve gear arrangements. The low mass and high stiffness of the system also permits valve lift velocities and accelerations which increase the area under the valve lift curve and thus provide increased specific engine output. Although other overhead cam configurations can be made to have comparable lift velocities and accelerations, a direct acting valve gear arrangement offers the additional advantage of permitting rotation of the cam contacting surfaces as the lifter rotates which is not possible with rocker arm type valve gear arrangements. Direct acting valve gear arrangements, therefore, allow higher permissible cam contact stresses.
In addition, the cam profile for other overhead cam valve gear arrangements with high lift accelerations and velocities is more complex than that required for direct acting valve gear. The simpler cam profile requirement of direct acting valve gear results in less manufacturing difficulties and less cost in the valve gear when high velocities and accelerations are desired.
Tappets for direct-acting valve gear are received in a guide bore provided in the engine above the combustion chamber and reciprocated therein in a film of engine lubricant provided to the guide bore. Tappets for direct acting valve gear must have a sufficient diameter to shroud the valve spring and provide adequate lift. Accordingly, tappets for direct acting valve gear generally have a length-to-diameter ratio of an order of magnitude of one. When the tappet bore is formed in cast iron, the body of the tappet may be formed from a suitable iron based material, or alloy steel, to match the hardness and thermal expansion properties of the guide bore.
It has long been desired to find a way to provide a tappet for direct acting valve gear of substantially lower weight than iron or steel and yet provide a tappet having similar durability and wear properties. Lower weight tappets permit greater valve acceleration for a given valve spring load.
Moreover, where the engine combustion chamber head is formed of aluminum it is desirable that the tappet for such a direct-acting valve gear application match the surface wear and thermal expansion properties of the aluminum engine head in order to prevent excessive oil flow at engine operating temperatures. Tappets of iron or steel possess the requisite durability and surface wear resistance, but exhibit a substantially lesser coefficient of thermal expansion. Thus, if the iron or steel tappets are optimally sized to the tappet guide bore when the engine is cold, upon the engine reaching normal operating temperatures, the tappet will fit loosely in the guide bore. Conversely, if an iron or steel tappet is optimally sized to fit the tappet guide bore in the aluminum engine head at normal engine operating temperatures, assembly at room temperature will be impossible because of an interference fit. Furthermore, if the assembly is performed with the engine head at normal engine operating temperatures and optimal clearances, the tappet will be seized in the guide bore upon the engine cooling after such assembly.
It has, therefore, been desirable to find a way to provide hydraulic lash adjusting tappets for direct-acting valve gear in engines having combustion chamber heads of aluminum or similar light weight high-thermal expansion type materials. It has further been desired to provide a hydraulic lash adjusting tappet for direct acting valve gear with engines having aluminum heads in which the tappet will be capable of operating against a cam shaft formed of hardened iron base material. This generally requires that the cam face of the light weight tappet be compatable in hardness and wear properties with the hardened face of the cam lobe. Furthermore, it has been desired to find a way to economically and conveniently provide in such an hydraulic lash adjusting tappet a controlled leak-down clearance, yet provide a light weight tappet body.